[time-nuts] quartz drift rates, linear or log
Tom Van Baak
tvb at LeapSecond.com
Sun Nov 13 08:05:58 EST 2016
Thanks for all the comments on this thread. Here is the first set of replies:
> Interesting, Tom. I don't think I see any of those pesky grain boundary
> shifts or readjustments in the lattice structure? If I remember, these
> can cause instant shifts in frequency that do not heal?
In this experiment I was more interested in long-term frequency. I think the frequency jumps you speak of may be at a finer level than I was measuring. They are easier to see if you use a TimePod and continuously collect data every second. They are harder to spot if you use a frequency counter and collect data once an hour.
That said, you can see that CH03 exhibits more spikes than any of the others and it will be set aside for a closer look. The nice thing about this stage of my time nut hobby is that I am no longer looking for the best oscillator. Now oscillators that are weird are interesting to me.
> These look like textbook examples of random walk frequency modulation.
> As this is a random process it is not surprising that they look different
> for each oscillator.
Yes and no. Look again at:
Many of the plots differ in appearance and it's clearly not always random walk. Each oscillator's drift rate, short-term noise (hours) and long-term noise (days) contributes to the appearance of the plot.
But, when you apply a 10-day linear fit to each DUT, you get a new plot. Attached. Also at:
And then, yes, the dominant noise is very much random walk FM. Still, there is a variety in performance: some more noisy than others. This shows up nicely in the HDEV plots or drift removed ADEV plots. Those are important because when an OCXO is used in a GPSDO, the linear-fitted residual data is much more relevant than the raw data.
To put it another way, the first plot above makes you think some of these OCXO will make a far better GPSDO than others. But the second plot shows they may all be quite similar. The 2nd order term of the loop enables this.
> Do you know what caused the frequency jump of CH18?
Good eye. I think that was me doing EFC testing. I can check my notes. It's not the TBolt or OCXO.
> So, are you measuring OCXO stability or EFC stability?
I measured the 10 MHz coming out of the BNC connector. The TBolt's are free-running (no GPS, no disciplining) and DAC/EFC is forced to 0 volts to reduce any impact it may have.
I have also done EFC testing on each unit; that's a separate report for later. In this thread I mostly just wanted to show examples of log and linear drift and to convey that long-term (days, weeks) logarithmic trends are effectively linear trends over shorter-term (hours, days).
> Just out of curiosity, what is the age of each of these Tbolts? (i.e. date codes?)
These TBolt's are all from the original 2008 TAPR group buy. The Trimble date codes range from 2002 to 2005. These are US-units, not Chinese eBay imports.
> When the purpose is correcting a GPSDO local oscillator during holdover,
> it depends on how long one expects to trust the corrected frequency.
> Practical realities make it pointless to trust corrections longer than a
> day or so, if that long. At that scale, these all look pretty linear.
I agree. See especially the new linear-drift-removed plot I posted for Attila above (and attached).
> I think this is a general rule: when they make precision oscillators
> (USOs) for spacecraft, they make a fairly large batch (a couple dozen)
> to some intermediate level of assembly, and then they run them for a
> while and watch them, and from that, they pick the "good" ones. (where
> good is somewhat mission dependent).
Right. Many of us time nuts have done the same with 10811A oscillators. You just keep buying them here and there over the years, comparing them, and keep the unusually good one that's down in the low -13's.
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